BEIJING: China will keep up its years-long battle against smog to ensure "blue skies" and promote a "revolution" in clean energy, President Xi Jinping said on Wednesday at the opening ceremony of a key Communist Party congress.

Improving the notoriously toxic air across the northern regions of the world's second-largest economy has been a cornerstone of Beijing's economic and social policy in recent years.

China has ordered factories to cut output in a bid to enforce bigger emission cuts in coming months and avoid a repeat of the near-record levels of choking smog that enveloped key northern areas at the start of the year.

In the long term, it has also launched a series of measures to curb the use of coal, the nation's favourite fuel, and boost use of renewable power, like wind and solar.

The government will also take measures to improve rural areas by restoring soil and waterways, Xi added, as China moves to modernise its vast agricultural sector.